The present invention relates to a nonaqueous electrolytic solution containing therein magnesium ions, and an electrochemical device using the same.
A metal which is easy to emit an electron to become a cation, that is, a metal having a large ionization tendency is given as a material suitable for a negative-electrode active material as one of basic constituent materials of a battery. In a battery using a nonaqueous electrolytic solution, metal lithium is given as this example. The battery using metal lithium as a negative-electrode active material is structured in the form of a battery using a nonaqueous electrolytic solution based on a combination of various kinds of positive-electrode active materials such as an oxide and a sulfide, thereby being commercialized. Thus, that battery is mainly used as a power source of a small portable electronic apparatus.
In recent years, for the purpose of enhancing the convenience, miniaturization, weight lighting, thinning, and an increased high function of the small portable electronic apparatuses have been steadily advanced year by year. Along therewith, a small size, a light weight, a small width, and especially a large capacity are required for the battery used as the power source of each of these apparatuses. Therefore, it can be said that the larger a capacity (mAh/g) per unit mass or a capacity (mAh/cm3) per unit volume of each of the negative-electrode active material and the positive-electrode active material composing the battery is, the better.
Comparing the energy capacity per unit mass of the metal lithium with that of other metals, the energy capacity of metal lithium (Li) is larger than that of any of other metals, and thus is superior to that of any of other metals. For this reason, heretofore, many studies about the lithium secondary battery have been reported. However, the lithium secondary battery involves a problem in stability, and lithium is limited in terms of resources and is expensive.
An example of a study about a nonaqueous electrolytic solution system battery using magnesium (Mg), as a metal having a higher energy density than that of lithium (Li), as the negative-electrode active material has been reported as a next-generation high capacity battery (for example, refer to a Non-Patent Document 1 which will be described later).
Magnesium is abundant in terms of the resources, and is much more inexpensive than lithium. In addition, metal magnesium has a large energy capacity per unit volume, and has a higher energy density than that of metal lithium. Moreover, when metal magnesium is used in the battery, the high margin of safety can be expected. As described above, the magnesium secondary battery is the secondary battery which can cover the shortcomings of the lithium secondary battery. On the basis of these respects, at the present time, the development of the nonaqueous electrolytic solution battery using metal magnesium as the negative-electrode active material gains recognition as the next-generation high capacity battery. As with this example, metal magnesium and the magnesium ions are very promising materials as the electrode active material in the electrochemical device, and the electric charge carriers in the electrolytic solution, respectively.
The selection of the electrolytic solution is very important in designing the electrochemical device using metal magnesium and the magnesium ions. For example, not only water and a protic organic solvent, but also an ester class and a nonprotic organic solvent such as acrylonitrile cannot be used as the solvent composing the electrolytic solution. The reason for this is because when these solvents are used, a passive state film through which none of the magnesium ions are not passed is formed on a surface of metal magnesium. A problem about the formation of the passive state film becomes one of hindrances in putting the magnesium secondary battery into practical use.
An ether solution of a Grignard reagent (RMgX: R is either an arkyl group or an aryl group, and X is any one of chlorine, bromine, and iodine) has been known as an electrolytic solution which does not involve the problem about the formation of the passive state film and which can electrochemically utilize magnesium since long ago. When this electrolytic solution is used, metal magnesium can be reversibly precipitated and dissolved. However, an oxidation and decomposition potential of the electrolytic solution is as low as about 1.5 V relative to an equilibrium potential of metal magnesium. Thus, a potential window is insufficient for use of that electrolytic solution in the electrochemical device (refer to a in FIG. 1 of the Non-Patent Document 1 which will be described later).
With regard to the nonaqueous electrolytic solution not using the Grignard reagent, there are the Non-Patent Document 1, a Patent Document 1, a Patent Document 2 and the like which will be described later.
Firstly, the following description is given in the Patent Document 1, which will be described later, entitling “Nonaqueous Electrolytic Solution of High Energy, Rechargeable Electrochemical Cell.”
A nonaqueous electrolytic solution for use in an electrochemical cell is composed of (a) at least one organic solvent, and (b) at least one electrolytic solution active salt represented by a formula M′+m(ZRnXq-n)m. In this formula, M′ is selected from the group consisting of magnesium, calcium, aluminum, lithium and sodium. Z is selected from the group consisting of aluminum, boron, phosphorus, antimony, and arsenic. R represents a group selected from the following group, that is, the group consisting of alkyl, alkenyl, aryl, phenyl, benzyl, and amide. X is halogen (I, Br, Cl, F). m=1 to 3. When Z=phosphorus, antimony and arsenic, n=0 to 5, and q=6. When Z=aluminum and boron, n=0 to 3, and q=4.
In addition, the following description is given as Example 3 of the invention in the Patent Document 1.
An electrochemical cell was composed of a Chevrel phase cathode, a magnesium metal anode, and an electrolytic solution containing therein a Mg(AlCl2BuEt)2 salt in THF and was prepared. 25.7 mg of the cathode was made of a mixture of the Chevrel phase material in which copper was leached out, and which contained therein 10 wt % carbon black and 10 wt % PVDF as a binder spread out in a stainless steel mesh. A solution thereof was prepared from 0.25 mol of a Mg(AlCl2BuEt)2 salt in THF. The anode was a disc of a pure magnesium metal having a diameter of 16 mm, and a thickness of 0.2 mm. The cell was packed in a stainless steel “coin cell” shape provided with a paper separator made of a glass fiber. The cell was subjected to the circulation of the standard charge-discharge having a current density of 23.3 mA/g. A potential limit for the circulation was lied between 0.5 V in a perfectly discharged state, and 1.8 V in a perfectly discharged state.
The battery was continuously subjected to the circulation for three months or more. A circulation possibility having the excellent circulation is clearly obvious from FIG. 3 of the Patent Document 1, and cycles 340 to 345 are shown adjacent to first five cycles (cycles 1 to 5). The result of the circulation is kept strong throughout the experiments. A charge density obtained in each discharge is 61 mAh per gram of the cathode material.
In addition, a description about a potential difference dynamic behavior of a MgxMo3S4 electrode in a tetrahydrofuran (THF) solution of a Mg(AlCl2BuEt)2 is given in the Non-Patent Document 1. Moreover, a typical charge-discharge behavior of a Mg—MgxMo3S4 coin cell type battery (an electrolyte is 0.25 M of Mg(AlCl2BuEt)2 in THF) is shown based on a relationship between the number of cycles, and a specific discharge capacity (mAhg−1).
Next, the following description is given in the Patent Document 2, which will be described later, entitling “Magnesium Secondary Battery.”
The invention of the Patent Document 2 relates to a secondary battery in which a negative-electrode active material is a magnesium metal, and a positive-electrode active material is a transition metal compound which can carry out intercalation of a magnesium ion, and an electrolytic solution is composed of an electrolyte containing therein a compound including an atomic group in which an aromatic atomic group and one halogen atom are linked to a magnesium atom, and a solvent composed of an ether system compound liquid. It is said that a charge voltage can be set as being equal to or higher than 2.3 V with this secondary battery.
The electrolyte described above is preferably halogenophenyl magnesium (C6H5MgX(X═Cl, Br)). In addition, it is said that the electrolyte described above is preferably a polymer gel electrolyte containing therein C6H5MgX(X═Cl, Br), and a polyethylene oxide (PEO).
In Example in which a THF solution of C6H5MgBr is used as an electrolytic solution, a decomposition start voltage is about 3.8 V, whereas in Comparative Example in which a THF solution of Mg[Al(C2H5)2Br2]2 is used as an electrolytic solution, a decomposition start voltage is about 2.3 V, and the electrolytic solution is oxidized even at 2.2 V to be colored with brown. Therefore, it was made clear that with the secondary battery shown in Example, the charge at the high voltage is possible. In addition, it was shown that the electrolytic solution used in the secondary battery has a high decomposition voltage.
In a word, according to the invention of the Patent Document 2, the secondary battery is prepared in which the negative-electrode active material is the magnesium metal, and the positive-electrode active material is the transition metal compound which can carry out the intercalation of the magnesium ion, and the electrolytic solution is made the electrolytic solution containing therein the electrolyte containing therein the compound including the atomic group in which the aromatic atomic group and one halogen atom are linked to the magnesium atom, and the solvent composed of the ether system compound liquid, whereby it is possible to obtain the secondary battery which has the high margin of safety, is inexpensive, and has the high electric capacity density, and with which the high charge voltage is possible.
In addition, a study about a graphite fluoride as a positive-electrode active material for which a higher capacity than that of a molybdenum sulfide can be expected is reported (for example, refer to a Non-Patent Document 2 which will be described later).